The invention relates to a linear actuator having a motion conversion mechanism from rotation to linear motion.
Two supporting methods are typically used in currently known linear actuators, such as pneumatic and hydraulic cylinders. One uses a fixed support with one degree of freedom for the output shaft, and the other applies a swing support with two degrees of freedom for the output shaft. There are two types of mechanisms for the latter case, the so-called Trunnion mechanism and the Clevis mechanism. In a pneumatic or hydraulic cylinder, any member of the Trunnion mechanism or Clevis mechanism can fairly easily be equipped on the cylinder parts, such as cylinder end block or cylinder tube. This is since said cylinder parts are designed as force-bearing members.
Recently, linear actuators with electric motors are emerging. The motor rotates a screw, such as a ball screw etc., and a nut engaged with the screw moves linearly, so that consequently an output shaft connected to the nut also moves linearly. In this type of linear actuator, the motor is mainly positioned on a support block supporting the screw in the direction of the screw axis and is located on the other side of the output shaft.
In the current pneumatic or hydraulic cylinders, the Clevis mechanism, which is one of the swing support mechanisms, is used to be positioned on the far end of the cylinder from the output shaft. In case the same way of Clevis mechanism positioning is applied for the linear actuator with an electric motor, the Clevis mechanism should be positioned around the end of the motor. However, the motor is generally not a force-bearing member which should be designed strong enough to be able to support the reaction force to the output shaft from the operated load. Therefore, any member of the Clevis mechanism should be on the side of the motor end. As a matter of fact, linear actuators with electrically powered motor are generally not equipped with the Clevis mechanism. Accordingly, such linear actuators with electrically powered motor are generally also not equipped with the Trunnion mechanism.
Taking into consideration the applications of linear actuators with an electric motor, a swing support mechanism such as Trunnion mechanism or Clevis mechanism is indispensable.
It is an object of the present invention to provide a technically and also economically reasonable selection way for supporting the linear actuator to be capable of taking a swinging action.
The object is solved by the independent claim. Preferred embodiments are shown by the dependent claims.
According to the invention, a linear actuator comprises:
a motor,
a rotation-to-linear motion conversion mechanism having a screw and a nut,
a support block supporting an end of the screw, and
an actuator-side support member with a swing support mechanism (e.g., a swing axis or bearing portion in case of a Trunnion mechanism, or a swing support bearing portion or support leg of the swing support bearing portion). The actuator-side support member is part of the support block or an extension member of the support block or connected therewith. The swing axis of the swing support mechanism is located in the plane including the screw axis, and the force flow line inside the linear actuator is limited on the output shaft, the nut, the screw and the support block.
The effect of the invention is that in the electrically-powered linear actuator, the screw is used to be supported by a bearing or by bearings in radial or axial direction, so that the screw can rotate keeping the axis stable and does not move in the axial direction due to a reaction force from an operated load applied to the screw through the output shaft and the nut. This means that the support block supporting the screw end is playing a role of a xe2x80x98waist bonexe2x80x99 and represents a force-bearing member on the force flow line.
From this point of view, the members for keeping the function and shape normal, but not the force-bearing members, are the body members covering the mechanism of the screw, the nut, the output shaft, and so on, the members supporting the output shaft in radial direction, the motor cover, the cylinder cover, and so on.
When a swing support mechanism, such as Trunnion mechanism or Clevis mechanism, is used, the actuator-side support member of the swing support mechanism should be supported by a force-bearing member.
Consequently, the construction of the mechanism according to the invention, wherein the swing axis is included in the plane including the screw axis, provides the effect that no bending moment emerges on the screw and correspondingly on the other members than the force-bearing members, when a force is applied on the screw in the direction of the screw axis.
Therefore, the composition according to the invention is the most desirable from the point of view of dynamics.
According to the ideal supporting point selection method in the swing support mechanism (e.g. Trunnion, Clevis, and so on) of the linear actuator, the members required to be force- bearing members play a role for supporting reaction force of the load operation. Thus, a bending moment is not applied to light weight members other than the force-bearing members, and, consequently, an economical machine design with light weight members having the required minimum strength becomes available in the linear actuator being electrically powered and providing capability for small size, light weight and a minimum number of force-bearing members.